1. Field of the Invention
The present invention relates to a semiconductor process. More particularly, the present invention relates to a silicon oxide gap-filling process, which is suitably used to fill up a trench having a high aspect ratio (AR).
2. Description of the Related Art
As the semiconductor industry advances into the deep sub-micron generation, the active area isolation structure in a device under 0.18 μm is usually formed utilizing a shallow trench isolation (STI) process. In an STI process, a shallow trench is formed first on a substrate, and then a chemical vapor deposition (CVD) process is performed to fill up the shallow trench with silicon oxide. Since a sub-micron device generally has shallow trenches of a high aspect ratio (AR), high-density-plasma chemical vapor deposition (HDP-CVD) methods are usually used for silicon oxide deposition. In an HDP-CVD process, deposition gases and sputtering-etching gases, such as inert gases like argon and helium, are simultaneously used to perform deposition and sputtering-etching at the same time, so that voids will not be formed in the silicon oxide layer filled into the trench.
However, as shown in FIG. 1, when the width of a trench 110 on a substrate 100 is further scaled down, the silicon sputtered from one side of the silicon oxide layer 120 will redeposit on the opposite side. Thus, overhangs 130 are formed at the top corners of the trench 110 to cause a void 140 to be formed in the silicon oxide layer 120. Therefore, the etching/deposition ratio (ED ratio) of the HDP-CVD process is usually adjusted to above 0.2 to well separate the two facets of the silicon oxide layer over the trench, so as to prevent joint of the two overhangs and formation of a void, as described in U.S. Pat. No. 5,872,058. However, the ED ratio cannot be overly raised, or a corner clipping problem is adversely caused. As shown in FIG. 2, when a silicon oxide layer 220 is deposited with an overly high ED ratio, the substrate 200 at the top corners of the trench 210 will be clipped, and the active area is thus damaged.
Another method for solving the overhang problem is to change the species and percentage of the sputtering-etching gas. When the aspect ratio of the trench is lower than 3 in a process above 0.18 μm, the sputtering-etching gas is usually argon, and has a percentage of 30˜60%. While the process linewidth is scaled down to 0.13 μm and the aspect ratio of the trench is raised to above 3 correspondingly, lighter helium is used in replacement of argon, and the percentage of helium is reduced to under 13% to inhibit the redeposition effect.
However, when the process linewidth is continuously scaled down to result in an aspect ratio equal to or higher than 4, even the optimum sputtering-etching/deposition conditions usually cannot overcome the redeposition and void problem. Therefore, some deposition/etching/deposition methods are provided to solve the problem, as described in U.S. Pat. Nos. 6,030,881 and 6,335,261. 6,030,881 discloses a method that periodically adjusts the ED ratio to alternately perform deposition steps and sputtering-etching steps. U.S. Pat. No. 6,335,261 discloses a method that alternately performs deposition steps and etching steps, wherein each etching step includes a reaction stage and a desorption stage. Though the deposition/etching/deposition methods are capable of solving the void problem, they all suffer from tedious steps.